This invention relates to dry kilns for drying lumber and other wood products, and more particularly to a novel sticker by which wood products are spaced apart vertically in a dry kiln for the circulation of heated air.
A "sticker" is a generic term for a spacer employed to separate layers of lumber in the kiln drying process. Its purpose is to allow hot air to flow uniformly between layers of lumber and allow moisture to escape from the wood. The rate of drying is critical and is controlled through the establishment of drying cycles which vary stepwise in temperature and time at each temperature. Drying cycles may very widely, depending upon the type of wood, with maximum temperatures ranging from around 160.degree. F. for helmock to 260.degree. F. or higher for southern pine.
Stickers are normally about 1.5 inches wide, from 0.625 inch to 1 inch thick and either 4 feet or 8-9 feet long. The length depends upon whether a crib or unit stacking technique is used. Stickers are laid perpendicularly across each layer of lumber by automatic sticking machines or by hand, with spaces between the stickers of about 2-3 feet. The number of stickers employed for a typical stack of lumber for dry kiln drying thus may total over 1,000. The total weight on the bottom layer of stickers can exceed 60,000 pounds, yielding compressive stresses exceeding 1,600 PSI.
It is a generally accepted practice to utilize wood material for stickers. These range from clean Douglas fir to seasoned hardwoods, or end cuts of plywood and 16-ply laminated Douglas fir. The primary limitations and disadvantages associated with wood stickers are that they are subject to breakage and warpage and they take a heat set over uneven layers. Wood also embrittles after heat aging and it compresses and gets thinner over time. Knots in wood present weak points which result in premature breakage.
The lifespan of wood stickers varies widely, depending upon the handling techniques, the wieghts supported by them, the drying temperatures and the type of wood used. It is the experience at some mills that the average lifespan is only 3-5 cycles of use, while others report 7-8 cycles. Some mills utilizing laminated stickers report as high as 10-15 cycles.
The cost of stickers also varies with the type of sticker. For a 4 foot sticker, current prices range from about $0.15-$0.25 and for 8 foot stickers from $0.35-$0.65, with laminated stickers costing about $0.65-$0.85 cents. Accordingly, the cost of stickers per cycle of use varies from about $0.03-$0.10, depending upon the type of sticker and the particular lumber mill utilizing them.
Other forms of lumber spacing stickers have been proposed heretofore. Among these are the solid, resin-impregnated paper laminate sticker described in U.S. Pat. No. 3,900,957; the wire mesh stickers and cooperating wood stickers described in U.S. Pat. No. 4,199,870; and the sticker frames described in U.S. Pat. No. 4,663,860. All of these are characterized by excessive cost and complication of manufacture, use and storage. Moreover, like wood stickers, they cannot be repaired or reprocessed when broken.